"Agnostic" is related to the word "gnosis", which refers to a hidden knowledge, generally of a spiritual variety (though I've seen it used in other contexts. too). The "a" confers a lack thereof, so an agnostic is someone who does not have hidden, spiritual knowledge; it's come to mean "doubter" in a popular sense, but when I hear someone say that they are agnostic, I tend to think that they are people who aren't schooled in the philosophy and principles of a given religion.
In my experience, most people who claim to be atheists have put a great deal of thought into it. It's still something of a shocking thing to say out loud (which isn't quite like typing a contribution on a message board). A real, genuine atheist is usually pretty quiet about it.
IMO, most of the vocal atheists, especially those who delight in blasphemy, really do believe in God because they are in a sort of constant argument with Him. Their very denial is, in a sense, perpetuating this argument and keeping it going.
My maternal grandmother was an atheist, but never discussed it. As near as I can tell, any spiritual beliefs that my mother ever learned came either from her grandmother or her father (who my grandmother insisted renounce his Catholicism before agreeing to marry him). I spent whole months of my childhood in their care, and not once was God or any kind of spirituality ever mentioned, let alone discussed. .
My paternal grandfather, on the other hand, claimed to deny the existence of God but never shut up about it and blasphemed constantly, just like he'd openly berate my grandmother (his wife) openly for going to church (she was the daughter of a Unitarian minister). But my grandfather was a bitter, hateful old man
My mother, who claims to be agnostic, hedged her bets. She raised my sisters and I to believe in God and my parents went church shopping before deciding they liked low-church Episcopalian best. Aside from occasional holidays, my mother only stepped into a church to attend weddings and funerals. She cannot bring herself to say that she has no faith, and she still (40 years later) "blames" Him for the sickness that led to my youngest sister's autistic-like condition.
When younger, my other sister, two years younger than me, claimed either agnosticism or atheism depending on her mood. In her 30s, she flirted with Wicca, but, due in no small part to the influence of her current partner, seems to have embraced a kind of church-free Catholicism, complete with a Mary mania that is incomprehensible to me. Her e-mails are full of such sugary piety that I find it difficult to believe that she wrote them.